Those skilled in the art will appreciate the harsh operating environment of communication devices such as mobile radios. The major contributors to a severely noisy environment for the mobile radio include engine noise, (both from the vehicle using the mobile radio and surrounding vehicles), electrical interference from high power lines, and atmospheric disturbances.
Some mobile radios have employed noise blankers to suppress or eliminate these noise effects. The basic purpose of a noise blanker is to detect the presence of impulse-type noise and momentarily prevent the noise in the recovered signal from reaching the intermediate frequency (IF). For the noise blanker to function properly, it must detect the presence of noise and inhibit the signal path in the main receiver before the noise gets to the point where it is to be stopped. Historically, implementation of a noise blanker in a mobile receiver was facilitated by the commensurate bandwidth of the main receiver and the noise blanker (i.e. each about 1 megahertz). Thus, the "race" condition was not a significant problem. Since the bandwidths were practically the same, the delay was effectively the same or could be compensated for by small "lump element" filters.
Modern mobile radios however, have extremely broad bandwidths. Since most mobile radios have frequency synthesizers that can generate a wide variety of frequencies, mobile radios today use broad bandwidth filters permitting the mobile radio user to operate over a wide band of frequencies. Thus it is common for a receiver to have bandwidth of 20 or 30 megahertz. However, this bandwidth extension creates significant problems in the operation of the noise blanker circuitry. Since the band width of the main receiver may be twenty times the bandwidth of the noise blanker (thus making the noise blanker delay 20 times that of the main receiver), control pulses can not reach the blanker switch in time to prevent the noise from entering the receiver IF. To compensate for a delay of this magnitude, a "lump-element" filter cannot be used since the current trend is toward radio size reduction. Hence, the size of such a filter would be prohibitive.
A solution to the delay problem was achieved using a surface acoustic wave (SAW) filter to afford both selectivity and time delay in an appropriately sized filter. However, SAWs are expensive commodities.
To further achieve miniaturization, microelectronic techniques are desired in fabricating radios. Receivers producing substantially low frequency intermediate frequency (IF) signals are known to be easier to implement microelectronically for the intermediate stage. Since this I.F. frequency may be substantially zero Hertz (i.e. DC or baseband), the term zero I F (ZIF) is used in describing such an IF signal or stage. "Direct conversion" receivers further utilizes the ZIF advantage to eliminate a prior stage by converting an incoming signal directly to baseband. With ZIF or direct conversion, the necessary sharp selectivity is then achieved through lowpass rather than bandpass filtering. Since low frequency lowpass filters are readily fabricated in monolithic form, a much greater degree of miniaturization can be achieved in proportion to the amount of bandpass filters being converted into lowpass.
Thus a need exists to provide effective noise blanking while contemporaneously providing broad receiver bandwidth and radio size reduction.